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O2< ^>-<^ >N2C1 + 2
Submitted by J. L. Ham well and L. F.
Fiesek. Checked by W. W. Habtman and G. L. Boomer.
In a i-l. beaker 2T.2 g. (0.1 mole) of o-tolidine is made into a thin
paste with 300 cc. of water and dissolved by the addition of 20 cc. (23.6
g., 0.23 mole) of concentrated hydrochloric acid (sp. gr. 1.18), warming
a little if necessary. The solution is cooled to ro° with ice, mechanical
agitation is started, and 21 cc. (25 g., 0.24 mole) more of concentrated
hydrochloric acid added (Note 1). This causes partial separation of o-
tolidine dihydrochloride. Keeping the temperature at 10-15°, a solution
14.5 g. (0.2 mole) of 95 per cent sodium nitrite in 40 cc. of water is
run in rapidly, holding back the last 10 per cent which is added slowly
and only as needed to give a distinct positive test for nitrous acid on
starch-iodide paper. This excess is maintained for one-half hour.
Throughout the diazotization an excess of hydrochloric acid should be
present as shown by a strong test
COUPLING OF o-TOLIDINE AND CHICAGO ACID 13
on Congo red paper. When the diazotization is complete, the excess
nitrous acid is eliminated by the careful addition of a small amount of
o-tolidine hydrochloride solution, using starch-iodide paper to arrive at
an exact balance (Note 2).
In a 3-I. beaker a paste is made from 82 g. (0.21 mole) of 88 per
cent technical Chicago acid (i-amino-8-naphthol-2,4-disul-fonic acid)
(Note 3) and 500 cc. of water, and solution effected by the addition of 8
g. (0.2 mole) of sodium hydroxide in 30 cc. of water, testing with litmus
toward the end and leaving the reaction still acid (Note 4). The solution
is cooled to 180 by the addition of ice, and just before the coupling 35
g. (0.33 mole) of anhydrous sodium carbonate is added.
With vigorous mechanical agitation the diazonium salt solution is run
into the Chicago acid solution rather rapidly. The blue dye which
separates at first, gradually dissolves on further stirring. After one-
half hour the solution is tested for alkalinity (Note s) E^nd for
completeness of coupling (Notes 6 and 7).
After stirring for a total of two hours, the mixture is heated to
85°, 15 g. of decolorizing carbon is added, and the solution stirred for
fifteen minutes and filtered. The filtrate is reheated to 8 s° with
agitation and the volume estimated. For each 100 cc. of the solution 27
g. of hydrated sodium acetate (usually 450-500 g.) is added slowly, with
vigorous stirring, in four or five portions (Note 8). The dye is thus
caused to separate in an easily filterable form. On " spotting " a drop
of the mixture on filter paper by means of a stirring rod, the dye should
form a dark blue mass in the center of a red or violet rim (Note 9). The
mixture, while still warm, is filtered with suction on a 20-cm. Buchner
funnel, and as much as possible of the mother liquor is removed (Note
10). The filter cake is dissolved in 1200 cc. of water at 85° and
resalted as above with about 300 g. of sodium acetate, in several
portions, or until most of the dye is salted out. In a " spot " test on
filter paper, little blue should be left in the rim. The product is
collected, redissolved in 1200 cc. of water at 85°, and salted again.
This time enough sodium acetate should be added to give a small rim which
shows no red impurity (Note 11). The product is collected, dried in an
oven at no° for at
least twenty-four hours, ground, and passed through a 40-mesh sieve.
The sodium acetate is then extracted by digesting the sifted dye for
ten to fifteen minutes with successive 500-cc. portions of boiling 95 per
cent ethyl alcohol, filtering with suction after each extraction, and
washing the product on the filter with a little alcohol (Note 12). After
four alcohol extractions the filtrate should give a negative test for
acetate. The product, which changes in appearance from bronze to green
during the extraction, is dried in air at room temperature. The yield is
80-81 g. (83-84 per cent of the theoretical amount) of dyestuff free from
inorganic salts and organic impurities (Note 13).
1. If the hydrochloric acid is added all at once instead of in
two portions as described, a solid will be obtained consisting of o-
tolidine coated with its dihydrochloride and the diazotization will occur
2. If a clear solution is not obtained at this point, any solid
may be removed by filtration.
3. Owing to the fact that this intermediate is a technical